By Andrew Hobbs
CHANGES to risk assessment methods could change the way process engineers plan for Gas Hydrates – the ice-like solids that canshut down subsea systems when they form.
Speaking with Oil & Gas Australia, Associate Professor of the University of Western Australia’s (UWA) School of Chemical and Mechanical Engineering Zachary Aman said many of the methods used to assess the risk of hydrate formation were out of date.
Early developers of risk assessment had looked for a way for process engineers to find the risks of hydrate formation on a desktop computer in a short space of time, he said.
To enable these calculations to be made more quickly, risk modellers had over-simplified the way that the risk of hydrate formation was calculated – making broad assumptions about how hydrates were formed.
“So one of the simplifications was to assume that the hydrate particles, when they’re in oil, are distributed much like sand in the ocean… when in reality we know that these particles are erratic – that they form these large aggregate
structures,” he said.
“It is not something that we can just dismiss.”
Dr Aman said the problem had historically been an academic one – with risk modellers saying that a system was at risk of hydrates forming when a critical boundary was crossed.
“Where in reality we don’t have enough data to inform those boundaries,” he said.
Today computers have advanced to the point where more data can be mapped and better models formed, but Dr Aman said despite these advances, it was still impossible to assess how much hydrate was in a pipeline at any one time.
As a result, researchers from UWA and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) had put efforts to calculate the presence of hydrates themselves to one side, Dr Aman said.
Instead, they were looking at the differential pressure of f lowlines and pipelines at any given moment, he said.
“If you have an oil and gas system, everything is driven by pressure – you have some kind of upstream capability of pressure you can deliver and you have some kind of downstream requirements,” he said.
But if hydrates are present in the mixture, the amount of energy required to move the mixture through the pipeline goes up, he said.
“So what it is trying to do is model how that fluid and that very complex mixture behaves relative to the pressure drawing force you have available to move it,” Dr Aman said.
“If it is so sticky or so viscous that it doesn’t want to move, that is when you cross the risk boundary.”
“The simplest way to put it is to say – how much of that expendable energy to drive the fluids are you going to burn through because of hydrate? If that number reaches 100 per cent you have a problem.”
“If you need more pressure differential than you have, that is when it becomes a flow assurance risk and that is the basis on which we are moving forward to try and produce these risk based models for industry,” he said.
These models could be used to develop a design tool – enabling process engineers to develop a system of flowlines well within the limits of acceptable risk for hydrate formation.
It could also be used as a diagnostic tool, Dr Aman said.
“This is part of the flow assurance engineer’s toolkit that we hope will enable them to do deeper analysis from a diagnostic perspective.”
In turn, this could help project developers determine flowline running times and insulation requirements, as well as the types and quantities of chemicals which may need to be injected into the flowlines.
Dr Aman said all data used in project calculations was available for scrutiny by any member of the engineering community – giving them the ability to form their own judgements about the quality of the model.
Researchers’ perspectives had also been shifted through UWA’s Flow Assurance Workshop for industry, where up to 40 industry personnel, including experts from Chevron and Woodside, heard from lead PhD students involved in the project, giving their feedback on the approaches taken, he said.
“[We look at] what we need to be doing differently that informs our outputs that then changes the model outcomes and the direction of the development.”
Work on the project was ongoing, Dr Aman said, with the broad partnerships researchers enjoy enabling them to develop stronger models.
“We have made a phenomenal amount of progress in the past 24 months,” Dr Aman said.
“But every time we unlock a new level, new obstacles emerge to get to the next one, so it is kind of this unfolding story that we are only beginning to understand.”